It is common practice to provide in an automobile distributor, automatic means for advancing the firing point of each cylinder ahead of "top dead center", that is, the point at which a maximum amount of burning of fuel in the cylinder occurs. This automatic means causes the amount of advance, which is commonly measured in degrees, to vary in accordance with the engine speed and other engine operating parameters. A timing light measures the amount of advance at a given engine speed. It has a strobe light in the form of a gas-filled tube that is triggered by pulses corresponding to sparks for a selected cylinder. The timing light is aimed at the engine block and the adjacent rotating flywheel thereon. A timing mark on the fly wheel appears stationary because the strobe cycling matches the engine speed. To measure advance, flashing of the lamp is delayed with respect to the spark event. A knob on the device is rotatable to adjust such delay so that the flash from the strobe light occurs at top dead center. The operator can then read the knob indicia to determine the amount of advance, which is precisely equal to the delay. One such timing light is disclosed in a patent assigned to the assignee of the present application, U.S. Pat. No. 4,095,170 for "Meterless Ignition Advance Measuring Device for Internal Combustion Engines." Other timing lights have an associated meter to measure advance. Still others do not measure advance at all.
It is important that the reading on the knob or the meter, in the case of advance-measuring timing lights, be accurate, that is to say, if the meter or knob reads "45.degree.", then there must be in fact a delay of 45.degree.. After determining precisely the magnitude of advance the timing light can be adjusted so that the displayed value is accurate. In the case of a non-advance measuring timing light, a device for determining advance can be used to see if the advance is truly 0.degree. (or within an acceptable range of say .+-.0.3.degree.). U.S. Pat. No. 4,063,152, entitled "Method and Apparatus for Timing Light Calibration", deals with a device that determines the advance of a timing light. Such determination occurs by utilizing ignition pulses to trigger an oscilloscope. The flashes are sensed by a photoelectric sensor and applied to the vertical input of a cathode ray tube. The location of the spike on the face of the cathode ray tube indicates the amount of advance or retard between the flashes and the ignition pulses. This device has the capability of testing the timing light at only one of two engine speeds, namely, 750 rpm and 3,000 rpm. Furthermore, the patented device includes two separate elements: a console and cathode ray tube structure, making it unduly cumbersome. Finally, "eyeballing" a cathode ray tube does not yield sufficiently accurate measurements to insure the accuracy of the timing light.